INSECTS : WIKGS AND THEIE APPENDAGES. 95 



it •some modifications. "WTiether, in tlie beetles that 

 fly "witli noise, the elytra [or wing sheaths] contribute 

 more or less to produce it, seems not to have been 

 clearly ascertained ; yet since they fly with force as 

 well as velocity, the action of the air may cause some 

 motion in them, enough to occasion friction. "With re- 

 spect to Diptera, Latreille contends that the noise of 

 flies on the wing cannot be the result of friction, because 

 their wings are then expanded ; but though to us flies 

 seem to sail through the air without moving these or- 

 gans, yet they are doubtless all the while in motion, 

 though too rapid for the eye to perceive it. When the 

 aphidivorous flies are hovering, the vertical play of 

 their wings, though very rapid, is easily seen ; but 

 when they fly off it is no longer visible. Kepeated ex- 

 periments have been tried to ascertain the cause of 

 sound in this tribe, but it should seem with different 

 results. De Geer, whose observations were made upon 

 one of the flies just mentioned, appears to have proved 

 that, in the insect he examined, the sounds were pro- 

 duced by the friction of the root or base of the wings 

 against the sides of the cavity in which they are inserted. 

 To be convinced of this, he affirms, the observer has 

 nothing to do but to hold each wing with the finger 

 and thumb, and, stretching them out, taking care not 

 to hurt the animal, in opposite directions, thus to pre- 

 vent their motion — and immediately all sound will 

 cease. For further satisfaction he made the following 

 experiment. He first cut off the wings of one of these 

 flies very near the base ; but finding that it still con- 

 tinued to buzz as before, he thought that the winglets 

 and poisers, which he remarked were in a constant 

 vibration, might occasion the sound. Upon this, cut- 



